Thanks for answering!
I started off using antibacterial soap, until the bump appeared and my piercer told me to switch to sea salt.
One more thing - the inside of the stud often twists down, so the 'stem' is visible in the inside of my nose. I asked my piercer if that was a problem, and he said it wasn't. Do you think it may have to do with this? I never touch the piercing except when cleaning.